(1) Field of the Invention
This invention relates to double walled insulated tubing and to a method of installing such tubing in a well, such as a geothermal or oil well.
There is requirement to provide excellent thermal insulation properties in a tubular pipe string where it is required to bring warm or hot liquids to the surface from a deep depth. In other words, it is required to keep the liquid cooling on its traverse from the bottom of the well to the top of the well to a minimum, even though the temperature of the surrounding formations may drop by an average of3.degree. C. per 100 meters. Such applications are as follows:
1. Geothermal wells which produce hot water from deep aquifers at rather low flow rates where the thermal water would otherwise cool a considerable amount on the traverse to the surface, thereby losing thermal energy available to surface consumers. PA1 2. Closed loop geothermal wells where thermal heat exchanger liquid, which is commonly water, is pumped down to the bottom of a well in an annular space formed between a cemented casing string and a tubing string while drawing thermal energy from the surrounding formations and subsequently transporting the thermal energy to the surface through the tubing string. PA1 3. Oil wells producing crude oil with a high bitumen or paraffin content. Without an efficient thermal insulation, the oil in the tubing string cools considerably as the oil flows towards the surface installation. Once the temperature drops below a level which is specific to the type of oil, bitumen or paraffin starts to change to the solid state and adhere to the internal wall of the tubing string. As a result, flow resistance increases due to the decreased cross-section of the tubing so that sucker rods may become stuck and break. To pump cool and, therefore, highly viscous oil through the pipeline sections close to the well head requires an unnecessary high amount of energy. PA1 4. To produce oil that is already highly viscous in the downhole oil bearing formation where hot steam is pressed into the formation around the well bore to heat up the viscous oil and, thus, improve its flow behaviour. Heat losses on the way to the surface are required to be low so as not to cause any unnecessary flow pressure drop in the production tubing string. PA1 providing a first length of outer tube with sealing means at a lower, in use, end thereof for preventing liquid ingress into said outer tube, PA1 mechanically connecting a second length of outer tube to the end of said first outer length remote from said sealing means to form an outer tubing string, PA1 suspending said outer tubing string from support means, PA1 locating first and second connected lengths of inner tube forming an inner tubing string inside said outer tubing string, said first and second inner lengths of tube being mechanically connected together, and said inner tubing string being spaced from said outer tubing string to provide a thermal insulation gap between the inner and outer tubing strings, said inner and outer tubing strings being discrete and separate from one another over substantially the whole lengths thereof. PA1 Advantageously, to equalise the hydro-static pressure against the outside of said sealing means in a well bore, the internal tube string is filled with a liquid and said sealing means is subsequently de-activated, for example pumped open.
(2) Description of the Related Art
Pipe strings that are covered with glass or stone wool and then wrapped with foils or strips of thin steel sheets to shield the insulating layer against water are frequently used for surface applications. However, water may enter the insulating material through small holes or cracks that may occur in the surrounding cover and reduce the insulating properties of such a string. Therefore, this known method cannot be used to thermally insulate production tubing strings exposed to high pressure liquids in the annular space between the tubing string and a deep wells' casing.
Pipe made from fibreglass has a lower thermal conductivity than steel and is usually used for pipelines or tubing strings for corrosive media rather than for the purpose of thermal insulation because the thermal properties are usually insufficient. Temperature limits and lower strengths further reduce the possible range of applications of pipe made from such material. Fibreglass material is also considerably more expensive than steel pipe of the same diameter and cannot be used in oil wells equipped with reciprocating sucker rods. Also, inventory costs would rise if fibreglass tubing has to be kept in stock in addition to the standard steel tubing that is generally used in oil field operations.
Hitherto, thermally insulated injection and production tubing strings have sometimes been used to complete steam injection wells to obtain increased thermal efficiency of the system. The strings, which often have a length exceeding 1,000 meters, are constructed from individual lengths of double wall pipe, each of which usually has a length of about 9 meters--that is equivalent to range 2 API tubing joints--to be run and retrieved by oil field rigs in the same way as single wall strings.
The most common version of such a string is shown in the Composite Catalogue of Oil Field Equipment, Volume 1, page 988H, 35th revision, 1982-83, published by Gulf Publishing in the U.S.A. The prior art tubing string will now be described with reference to FIGS. 1 and 2 of the accompanying drawings in which FIG. 1 shows a longitudinal cross-section of a tubing string and FIG. 2 shows an enlarged, detailed, longitudinal cross-sectional view of a part of the string shown in FIG. 1 to more clearly show the connection between two lengths of tubing.
The thermally insulated tubing shown in FIG. 1 depicts one individual length of thermally insulated tubing having connections at opposing ends thereof, but it is to be realised that in practice there would be a plurality of such tubes lengths used to form a thermally insulated tubing string. FIG. 2 shows constructional details of the threaded connections used to form a length of insulated tubing connected one to the other.
In FIGS. 1 and 2, an internal tube 1 has the internal diameter thereof increased at opposing ends and the internal tube 1 is positioned concentrically inside an external tube and the opposing ends of the internal tube are welded to the external tube 2. Located in the annular space 3 formed between the external wall of the internal tube and the internal wall of the external tube is an insulation medium such as air. The external tube 2 is slightly longer than the insulated double wall section so that connecting screw threads can be cut at opposing ends of the external tube. Double box couplings 4 are used to link one length of dual insulated tubing to the next dual tube in an axial direction. So as to avoid internal diameter changes at each joint connection in the string, which would undesirably increase dynamic pressure losses in the flow of fluid, e.g. oil, non-insulated section sleeves 5 are inserted at the tube ends at the time of running the string into the well.
Mechanically, such strings fulfil all the strength requirements for deep wells. However, even in the ideal case where liquid from the well bore does not enter into the annular space 6 between the sleeve 5 and the box coupling 4, a considerable amount of thermal energy is lost through the thermally conducting bridges created by the internal and external tubes being welded together and also by there being a mechanical connection between the sleeve 5 and the box coupling 4. However, water or oil will usually enter the annular space 6 to further reduce the efficiency of the insulation. Any mechanical damage to the internal tube 1 caused by reciprocating sucker rods or by corrosion will permit liquid to flow into the annular space 7 between inner and outer tubes, thereby causing a thermal bridge that will not immediately become visible at the surface since the annular space is closed off at the end of each internal tube by the internal tube being welded to the external tube.
Another short coming of this type of insulated string is the requirement to weld the internal and external tubes together. In the drilling industry, it is usual to avoid, wherever possible, welding tools or equipment that is positioned downhole because such welds are the starting points for corrosive leaks. Thus, in the present thermal insulated tubing string example, welding between the internal and external tubes may well be the starting point for a corrosive leak. If corrosion starts from inside the annular space 3 between the two tubes, it will neither be detected visually nor by non-destructive inspection methods commonly used in the oil and gas industry.
When steel tubes leave the production line of a factory they normally have wide length tolerances. So as to match internal and external tubes of dual wall tubing, tubes have to be cut to matching length, thus increasing material cost. The main reason why dual wall tubing strings are not more frequently used is the high cost of purchase, being a multiple of the cost of plain tubes, as well as the increased delivery times for the especially manufactured welded joints and, lastly, but not least, increased inventory levels.
Another dual wall pipe designed for thermal insulating purposes is disclosed in EP-A-0138603. This reference overcomes the difficulty of the above-mentioned prior art by providing a passageway between the connection of two connected lengths of tubing so that the annular space in the respective lengths of tubing are connected together. Thus, in EP-A-0138603 the ends of the internal and external tubes are connected together by a wall and a small passage having a smaller cross-sectional area than the cross-sectional area of the annular space extends longitudinally axially from the annular space outwardly of the conjunction of the internal and external tubes. Because the location of the passage in each tube may not exactly align with one another, so the longitudinally axially outermost portion of the passage is enlarged in cross-sectional area so as to ensure that when two lengths of dual walled pipes are connected together, their respective passages will be interconnected. At an internal and external side of the passage where the two lengths of tubing are connected together there is provided respective annular seals to create a pressure seal between the annular space inside the insulated tubing string and both the liquid inside and around the insulated string.
The annular space may be filled with any desirable insulated gas or liquid and may, alternatively, be evacuated from the surface. Thus, the interconnected annular space can be used to check for leaks in any of the seals or in the walls of the tubing. If the annular space is initially filled with gas, a leak is indicated by an increase in pressure which will rise until the pressure in the annular space balances the pressure of the liquid, either inside the insulated string or outside of it. Once again, this reference has the disadvantage of the above-described insulated dual wall tubing and if either the internal or external tubing should have a leak, the leaking substance will extend throughout the annular space of the whole string. In such an event, it is difficult to locate the position of the leak which may be in elastomer seals radially inward and outward from the passage.
Another known dual wall pipe for reverse circulation drilling is disclosed in GB-A-1204026. In this reference, two concentric tubes are connected to one another by fins that are welded in the annular space between the internal and external tubes. The internal tube is recessed into each end of the external tube and the external tube is provided with a screw thread for connecting individual lengths of tube together. When a string is run into a well, a bridging sleeve having seals at opposing ends thereof is inserted over the internal tube of the upper and lower string section to seal the internal tubes of two different dual wall sections together. Thus, the sleeve extends into the next tubing joint where it is sealed against the internal tube of the next section. The requirement for sleeves connecting the internal tubes to one another adds to the cost of purchasing, storing and maintaining the string. However, a dual wall tubing constructed in accordance with this reference also has the disadvantages mentioned above in that the internal and external tubes are mechanically connected together, thereby creating a thermal connection between the inner and outer tubes.
A thermally insulated pipeline for transporting liquids and gas over the Earth's surface is disclosed in WO91/19129. This reference discloses two concentric steel tubes spaced by an annular space and in the annular space is an insulating material formed of micro-glass fibres or micro-mineral fibres having a compressive strength sufficient to keep the inner tube suitably spaced from the outer tube. The insulating material in the annular space is required to carry the weight of the internal string, since the tubular strings are approximately horizontal, without the insulating material losing its insulating properties. So as to have the appropriate insulating properties, solid insulators are referred to which are extremely porous. These support insulators add to the cost of the tubing string but, even worse, liquids which may pass into the annular space through a leak in one of the internal or external tubular strings will enter the pores of the insulating material of the insulators so that the material has to be replaced.
There is no disclosure in this reference of running such a string of generally independent concentric tubular members designed for more or less horizontal pipelines into a liquid filled, generally vertical, borehole or well and where the annular space between the internal and external tubes would have to be sealed against the influx of liquids contained in the well.
Oil field equipment is not designed to simultaneously run or pull concentric strings of pipe with differing diameters. Oil field tubing joints, according to the worldwide accepted API standard, do not have uniform lengths and joint lengths fluctuate considerably. Only with non-standard, more expensive tubes machined to identical lengths, can concentric strings of pipe be run simultaneously in a slow operation. Although above the surface pipelines are usually welded because at any time access can be obtained from the surface at any point along the line, in distinction, downhole tubing strings are usually threaded since, otherwise, the string has to be cut into slices every time it has to be pulled from the well for any reason.